Processes for coating aluminum and alloys thereof



United States Patent F PROCESSES FOR COATi'NG ALUMINUM AND ALLOYS THEREOF Jos Patrie, Tonrnus, France, assignor to Pechiney Compagnie de Produits Chimiques et Electromtallurgiques, Paris, France, a corporation of France No Drawing. Application March 11, 1952, Serial No. 276,047

Claims priority, application France March 16, 1951 Claims. (Cl. 20438) in general, the products obtained according to the known processes present various defects, the most serious of them being: insufficient adherence, blisters, lack of homogeneity, porousness of the layer of metal deposited, sensitiveness to corrosion, etc.

For example, in the commercially highly important instances of nickel and/ or chromium coating, where it is generally necessary to deposit first a layer of zinc, then another layer of copper before nickel plating, the processes currently employed in the art retain the above defects. The presence of an intermediate layer of copper is largely responsible for this state of affairs.

On the other hand, when it is desired to obtain bright deposits of nickel or of chromium, it is necessary to elfect several polishings, especially before coating with zinc, following coating with copper and following coating with nickel; the expense involved is considerably increased and mechanization of the treatment is made possible.

The present invention has for its object a process which makes it possible to obtain on aluminum and its alloys, sound electrolytic deposists which adhere firmly, resist corrosion well, are free from blisters or other defects and, moreover, are capable of taking a very high lustre.

According to one feature of this invention, aluminum or an alloy of this metal, after having been degreased and pickled according to a known method, is treated with an alkaline zincate solution, containing an alkali cyanide and copper cyanide. This treatment, which has for a result the formation of a highly adherent, homogeneous, fine-grained zinc deposit, can be carried out electrolytically, but it is preferable to produce it by simple immersion.

While the alkaline zincate solution may be prepared in ditferent ways, it is best that its composition be comprehended Within the following limits:

ZnO to 150 grs./litre and preferably 50 to 100 gr./l.

NaOHl00 to 600 grs./litre and preferably 250 to 350 gr./l.

KCN to 170 grs./litre and preferably 130 to 170 gr./l. CuCN0 1 to 40 grs./litre and preferably 3 to 6 gr./l.

As is well understood, the caustic soda may be replaced by potash and the KCN by NaCN. The copper may be replaced in part or in whole by another heavy metal capable of forming a cyanide complexi. e. a double cyahide-for example, by silver or cadmium. The concentration of the CuCN should not exceed Mt that of KCN.

According to a second feature of the invention, particularly favorable results are obtained when the articles 2,745,799 Patented May 15, 1956 to be coated, having received a first treatment with an alkaline zincate, are subjected to the action of an acid which is capable of dissolving zinc and, thereafter, to a second treatment with the zincate. These various treatments are accompanied by the usual operations and rinsmg.

Another feature of the process, according to the present invention, resides in the use of slightly acid nickelplating baths having a high concentration of strong electrolytes, preferably sulfates, for nickel-plating electrolytically the articles which have been zinc-plated in accordance with the above described method.

Baths having the following compositions can be used successfully:

Gr./litre NlSO4.7H2O 70 to H3BO3 10 t0 NH4C1 or NaCl 10 to 30 NazSO4..10H2O 200 to 450 The sodium sulfate can be replaced in part or in whole by the corresponding salt of potassium, lithium, or of magnesium, but the molecular concentration of the strong electrolyte must always be greater than that of the nickel salt used.

These baths, having a pH between 5 and 5.6, can be used at temperatures between 15 and 40 C., or preferably, between 20 and 35 C. and with a current density of 1 to 4 amp./dm'. Under these conditions, there are obtained finer-grained, highly adherent nickel deposits of a quality impossible to obtain by the known methods on aluminum and its alloys. It is remarkable that the process 'gives excellent results even in the case of alloys which, in general,are difiicult to coat by electrodeposition 'as is true, for example, in the case of Al-Mg alloys.

The nickel deposits obtained in the manner described above, have, moreover, the advantage of not requiring laborious treatments, polishings, etc., when it is desired to cover them with a bright chromium layer. In contrast with the practices of the prior art, it is sufficient, when proceeding in accordance with the present invention, to lightly burnish, i. e. color (which termsare used here and in the appended claims to designate the last stage of a polishing treatment) the nickel layer before it is chromium plated; there are obtained in this way, deposits of chromium which have a brightdecorative appearance. Moreover, in the case of articles of simple shape, the coloring (burnishing) of the nickel layer is not eve necessary and the chromium plating can take place immediately.

According to another feature of the invention, articles of aluminumor aluminum alloys can receive a bright deposit'of electrolytic nickel having a strength and lustre superior to those obtained by the known methods and, in particular, by the electrodeposition of a bright nickel layer on a underlying layer of copper. According to this particular feature of the present invention, the bright nickel layer is deposited in the presence of brightening agents in accordance with'known techniques (for example, those described in the U. S. Patents Nos. 2,191,813; 2,238,861; 2,523,190; 2,523,191; 2,402,801; etc.) on a primary electrolytic deposit of nickel obtained, in accordance with the previously mentioned features of the present invention, in-a bath having a high sulfate content; this primary deposit is depassivated by treatment with an acid, and is rinsed before being subjected to plating with a bright nickel layer.

If the article is to receive a chromium finish, this may be done under the usual conditions, and there is obtained a decorative chromium plating of very high quality.

The zinc plating of aluminum or its alloys with an alkaline zincate solution in the presence of an alkali cyanide and of a heavy metal capable of forming a cyanide complexi. e. a double cyanide-constitutes, according to the present invention, a preparation of the surface to enable the deposition not only of ferrous metals, such as nickel, chromium, cobalt, but also of other metals such as cadmium, silver, gold, tin, etc. and also of alloys, such as bronze, brass, etc.

In the case of brass, particularly, when the latter has been deposited from a cyanide bath, according to a known method, on an article which has preliminarily been treated in accordance with the present invention, the highly adherent brass layer is adapted to serve as a support 7 for other deposits. Thus, very good results are obtained by depositing on the brass-coated articles, following a simple washing operation, a bright nickel layer by a known process. If a chromium deposit be desired, it may be applied and it has the advantages previously mentioned, resulting from operating in accordance with the present invention.

Example 1 Articles of Duralumin (that is, an alloy of aluminum containing approximately 4% Cu, 0.6% Mg, 0.6% Mn and 0.4% Si) are degreased first, by means of trichlorethylene and then, electrolytically in a bath containing 100 grams per litre of NaaPOt, gr./l. of NazCOz and 5 gr./l. of NazSiOa at 12 volts, 5 amp./dm. for one minute at 20 C. Following rinsing in running water, neutralization with an aqueous solution containing 15% by volume of H2804, and 5% by weight of H2CrO4, and washing, the articles are immersed for seconds in a solution maintained at 30 C. and containing:

Grammes per litre NaOH 300 ZnO 75 CuCN 6 KCN 170 As a result, there is formed a deposit of Zinc which is washed, then dissolved in nitric acid of Baum. After being washed, the articles are again immersed in the above described zinc bath for seconds at 30 C. They are then washed and subjected to electrolytic coating with nickel in a bath composed of:

Grammes NlSO4.7H2O 110 NaCl 25 Na2SO4.10H2O 200 H3BO3 Water 1000 .the pH of this bath ranging between 5.5 and 6.2. The operation is carried out at 20 C. with a current density of 1.5 amp./dm. for 15 minutes while the solution is energetically agitated by the injection of compressed air. The thickness of the nickel deposit on the article is approximately 5 to 6 nncrons; it is firmly adherent, homogeneous, and fine-grained.

Example 2 Example 3 The treatments described in Example 1 are applied to articles made of commercial aluminum, following which the nickel surface is depassivated by dipping the articles for 2 minutes at ordinary temperatures in an aqueous solution containing 300 gr. of H2804 per litre. Once rinsed, these articles are coated with a bright nickel deposit, according to a known process, in a bath composed of 300 gr. NiSOeJHzO, 60 gr. NiClz, 6H2O, 60 gr. H3303, 1000 gr. of distilled water and 10 gr. of brightening agents (described in U. S. Patent No. 2,191,813); this bright nickel plating is carried out at 55 C. with a current density of 4 amp./dm. for 10 minutes, while the articles are agitated mechanically.

7 Example 4 Example 5 Following degreasing and pickling in a bath containing a phosphate and carbonate of soda, articles made of an aluminum alloy containing 4.5% Si, 0.7% lg and 0.5% Mn, are rinsed and neutralized with a solution containing 15% H2804 and 5% H2C1'O4; they are then again washed, and are thereafter dipped for about one minute in 40 Be. nitric acid, to which has been added 0.5% HP.

The articles are then rinsed and coated with zinc in a bath having the following composition:

Grs./litrc NaOH 350 ZnO CuCN 4 NaCN The sequence of operations is the same as in Example 1 following the coating with zinc, and is completed by bright nickel plating, according to Example 3, then chromium plating in accordance with Example 4. There is obtained in this way, without resorting to any intermediate or final mechanical polishings, articles with a solid chromium coating and a high lustre.

Example 6 The operations described in Examples 1 and 3 are carried out on articles of an aluminum alloy containing 11% Mg, thereby producing a final nickel deposit which is bright and highly adherent.

Example 7 Articles of Duralumin are Zinc plated twice in the alkaline Zincate solution containing cyanides, the operations being carried out as set forth in Example 1; they then receive an electrolytic deposit of brass in a bath having the following composition:

CuCN 30 Zn(CN)2 8 5 NaCN 50 Nazcoa 30 Water 1000 The operation is carried out at 35 C. wtih a current density of 0.5 amp./drn. for 2 minutes. The resulting deposit is 0.5 micron thick, and is highly adherent.

Example 8 Articles which have been zinc and brass plated, as in Example 7, are directly coated by a known process (Cf. U. S. Patent No. 2,191,813) with a very thick-40 microns-electrolytic deposit of bright nickel. well protected against corrosion by this thickness of nickel, which is very shiny and does not show any blisters or other defects.

They are Example 9 Articles which have been zinc, brass and nickel plated, as in Example 8, are chromium plated in a bathmaintained at 35 C.containing 300 gr. HzCrOa and 3 gr. H2504 per litre, with a current density of 12 amps/dink I claim:

1. Process of electrolytically coating aluminum and alloys thereof with other metals comprising the steps of: zinc plating the article in an aqueous alkaline solution consisting essentially of zincate, alkali cyanide and a cyanide of a heavy metal of the group consisting of copper, silver and cadmium, wherein the ratio of weight of the alkali cyanide to the heavy metal cyanide is greater than 4, and thereafter electrically depositing a metal other than zinc.

2. A process according to claim 1, wherein the heavy metal is copper.

3. Process for electrolytically coating aluminum and alloys thereof with other metals comprising the steps of: zinc plating the aluminum by immersion in an aqueous alkaline solution consisting essentially of zincate, alkali cyanide and a proportion by weight of copper cyanide less than A that of the alkali cyanide; dissolving by an acid the layer of zinc thus formed; again zinc plating and thereafter electrolytically depositing a metal other than zinc.

4. A process according to claim 3, wherein the metal other than zinc is nickel, deposited from a bath containing, in addition to a solution of a nickel compound, a strong electrolyte in a molar concentration greater than that of the nickel compound in solution, the said bath having a pH ranging between 5 and 6.5.

5. A process according to claim 4, characterized by the additional steps of depassivating the nickel deposited by treatment with an acid solution, and electrolytic deposition of nickel in a bath containing a brightening agent, whereby a bright nickel coating is obtained.

6. A process according to claim 4, characterized by the additional steps of burnishing the nickel deposit, and electrolytically depositing chromium thereon in a chromic acid bath containing sulfuric acid.

7. A process according to claim 4, characterized by the additional steps of depassivating the nickel deposited by treatment with an acid solution, electrolytic deposition of nickel in a bath containing a brightening agent, whereby a bright nickel coating is obtained, and electrolytic deposition of chromium on the bright nickel coating in a chromic acid bath containing sulfuric acid, whereby an outer coating of bright chromium is obtained.

8. A process according to claim 3, wherein the metal other than zinc is brass deposited electrolytically from an alkaline bath containing an alkali cyanide and cyanides of zinc and copper.

9. A process according to claim 8, characterized by the additional feature of providing a bright nickel coating by electrolytically depositing the same from a bath containing a brightening agent.

10. A process according to claim 8, characterized by the additional features of providing a bright nickel coating by electrolytically depositing the same from a bath containing a brightening agent, and thereafter chromium plating electrolytically the nickel coating in a chromic acid bath containing sulfuric acid.

References Cited in the file of this patent UNITED STATES PATENTS 1,405,534 Merritt Feb. 7, 1922 2,091,386 Viers Aug. 31, 1937 2,142,564 Korpiun Jan. 3, 1939 2,418,265 Korpiun Apr. 1, 1947 2,496,845 Balden et a1. Feb. 7, 1950 2,650,902 Higgins Sept. 1, 1953 2,662,054 Passal Dec. 8, 1953 

1. PROCESS OF ELECTROLYTICALLY COATING ALUMINUM AND ALLOYS THEREOF WITH OTHER METALS COMPRISING THE STEPS OF: ZINC PLATING THE ARTICLE IN AN AQUEOUS ALKALINE SOLUTION CONSISTING ESSENTIALLY OF ZINCATE, ALKALI CYANIDE AND A CYANIDE OF A HEAVY METAL OF THE GROUP CONSISTING OF COPPER, SILVER AND CADMIUM, WHEREIN THE RATIO OF WEIGHT OF THE ALKALI CYANIDE TO THE HEAVY METAL CYANIDE IS GREATER THAN 4, AND THEREAFTER ELECTRICALLY DEPOSITING A METAL OTHER THAN ZINC. 